only print "default: " for configuration option of strlist type if default is not empty

[Flamefire]

This changes (type comma-separated list; default: ) to (type comma-separated list) similar to a default of None.

That changes the output of --help in the following places (done using diff over the output before and after):

                    Parse (additional) configfiles (type comma-separated list)
                    Enable dependency resolution, using easyconfigs in specified paths (type pathsep-separated list)
                    Additional paths to consider by robot for easyconfigs (--robot paths get priority) (type pathsep-separated list)
                    Additional locations to consider in --search (next to --robot and --robot-paths paths) (type pathsep-separated list)
                    Location(s) of extra or customized easyblocks (type comma-separated list)
                    Location(s) of extra or customized module naming schemes (type comma-separated list)
                    Location(s) of extra or customized toolchains or toolchain components (type comma-separated list)
--from-pr=PR#       Obtain easyconfigs from specified PR (type comma-separated list)
                    Include easyblocks from specified PR (type comma-separated list)
                    Accept EULA for specified software [DEPRECATED, use --accept-eula-for instead!] (type comma-separated list)
                    Accept EULA for specified software (type comma-separated list)
                    Silence specified deprecation warnings out of (python2, Lmod6, easyconfig, toolchain) (type comma-separated list)

BTW: A colon after "type" would probably be nice.

[jfgrimm]

seems sensible to me, given that we don't output the default None